Derby took the lead when Kornell McDonald's low cross found Louie Sibley who fired the ball low beyond Michael Verrips.
The hosts equalised when Kyron Gordon's cross was met by a header from Luke Freeman eight minutes after the break, before substitute Billy Sharp intercepted McDonald's weak back-pass and side-stepped Ryan Allsop before slotting into the unguarded net for the winner.
Jokanovic said: "It's important, especially after the start of the season. We expect to be at a better level.
"We didn't perform in the first 45 minutes. We moved the shape of the team, found a bit more quality in the second half and scored two goals.
"I can complain about some of the positioning and concentration, but I cannot complain about the effort.
"Billy Sharp was clever for a lot of years and still is clever. He's a really good option for us. He's still scoring.
"This is good for our mentality and it will help ahead of the Luton game."
Derby boss Wayne Rooney praised his side's performance - despite the result - and urged them to be more clinical.
He said: "I thought they were superb. The performance was very good.
"First half, we were by far the better team and in the second half we conceded two goals that should have been avoidable. We have to be more clinical.
"There were a lot of positives. There were young lads getting asked to step up earlier than they should be.
"For these lads to come in and put in that sort of performance, they deserve a lot of credit.
"Now our attention turns to Nottingham Forest on Saturday and we'll be ready for that game. They haven't had a great start to the season and we'll put pressure on them."